Lateral diffusion rates of oxygen in erythrocyte membranes and phospholipid dispersions (liposomes) will be measured by a fluorescent assay procedure. The quenching of pyrene fluorescence by oxygen can be shown to be accomplished by a diffusion-limited reaction between oxygen and the excited state of pyrene. Due to the extreme sensitivity of fluorescence analysis, this reaction provides an assay procedure for precise measurement of oxygen diffusion. The effect of varying membrane composition (i.e. high or low degree of saturation of fatty acids, high or low cholesterol content) on the oxygen diffusion rates will be evaluated in order to ascertain how changes in membrane composition during disease influences oxygen accessibility to the cell.